[migration] stop caching the mapping from constraint type name to eid
It's not so frequent that a cache seems necessary, and we were not
invalidating that cache when adding a new constraint type.
Related to #3724892.
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s entity's classes"""